Before you start...

energy, energy units, and the difference between kinetic and potential energy

review the relationship between temperature and average molecular velocity

Lecture outline

In all chemical change, chemical bonds are broken or formed.
Energy is required to break a chemical bond (just as energy is required to stretch a spring until it breaks).
Conversely, forming a chemical bond releases energy. Virtually all chemical reactions absorb or release energy because
bond making seldom exactly balances bond breaking in the reaction. In this unit, we will learn to measure and predict the amount of heat absorbed or released by a chemical reaction.

The concept of energy

the usual definition of energy: the ability to do work

work is moving an object against an opposing force

work = distance × opposing force

SI unit of work or energy: the joule (J)

two basic forms of energy

potential energy: energy of position

examples

boulder on a ledge

cations and anions

chemical bonds

kinetic energy: energy of motion

examples

pool balls

molecules

why is the concept of energy useful?

if something is isolated from everything else, its total energy never changes

this allows seemingly unrelated behaviors of the system to be connected

example: the pendulum

Two things energy is NOT

some sort of invisible fluid

something which can be measured directly

Thermal energy

definition: energy due to chaotic molecular motions

three factors affecting thermal energy

temperature

higher temperature leads to higher thermal energy

sample size

a cup of hot coffee has more energy than a teaspoon of coffee, all other things being equal.

composition

E(solid) < E(liquid) < E(gas), all other things being equal

anything that changes temperature, sample size and/or composition of an object can change its thermal energy

Heat

definition: transfer of thermal energy due to a temperature difference

thermal energy isn't measurable, but heat is

Three factors affect how much heat an object absorbs or loses

mass of object

temperature change of object

final temperature - initial temperature

if there is no change in temperature, no heat flows

composition of object

specific heat: heat required to raise the temperature of 1 g of material by 1 K

different materials have different specific heats

material at298 K and 1 atm

specific heat(J/g K)

ice

2.09

water

4.18

steam

1.86

sodium

1.23

aluminum

0.9

iron

0.45

heat capacity: heat required to raise the temperature of an object by 1 K